Biomedical Engineering Reference
In-Depth Information
One such system is solutions of xanthan, a microbial exopolysaccharide, discussed in
Chapter 5
. In steady shear measurements, apparent viscosity shear rate power-law
exponents are high (
le is more
like that of a so-called Bingham or viscoplastic material (Goodwin and Hughes,
2000
)
rather than a pseudoplastic
≈
0.9), and the overall viscosity versus shear rate pro
0, the
viscosity levels off to give the constant Newtonian viscosity, while for viscoplastic
systems the apparent viscosity
fluid. For the pseudoplastic material, as the shear rate
→
η
→
∞
(Richardson and Ross-Murphy,
1987
).
log) viscosity
versus shear rate curves have, nevertheless, been reported in a number of other systems,
especially including those having long-range ordering and structure. The converse
behaviour, in which
Failure of the Cox
-
Merz rule, and the observation of power-law (log
-
η
lies above
η
*
, is seen in certain hydrophobically modi
ed polymer
solutions and also in a number of micellar
'
gels
'
. Such behaviour is predicted by the
Tanaka
Edwards theory (Tanaka and Edwards,
1992a
,
1992b
,
1992c
) and is presented in
Chapter 6
in terms of the short and reversible network lifetimes; no current theory seems
able to predict the
-
η
<
η
* behaviour, although it is intuitively easier to understand.
2.5.2
Large-deformation measurements
Many published large-deformation measurements take a very simplistic approach in that
they make cylindrical samples of gels and simply crush them between two cylindrical
plates. The force and amount of compression are measured, and various signature traces
can be obtained. This is the basis of the technique of
used in quality-
control laboratories. In such compression measurements, the usual con
'
texture analysis
'
guration is a
vertical movement motor and
fixed normal force transducer, and this is used in many
instrument designs (Ross-Murphy,
1994
). Although originally these were quite bulky, a
number of miniature versions are available and are probably more useful for gel systems.
However, the problemwith this approach is that in compression the point of failure of the
sample can be quite dif
cult to establish simply from the trace obtained, and much of the
compression involves further maceration of an already ruptured sample. Interpretation of
such results in terms of gel macromolecular properties is almost impossible. The surface
properties of the gel and the interaction with the (metal) interface of the compression rig are
also crucial, and results may be quite different if the gel surface is lubricated.
A much cleaner and more easily interpretable approach is to measure in tension, rather
than in compression, but there are problems there as well. One approach is to
'
superglue
'
the top and bottom surface of the gel to the instrument and then set the instrument to
'
on the gel. This is more reliable that using the usual (dumbbell-shaped) solid samples. In
either case an external extensometer needs to be employed, since the samples tend to
'
'
pull
in the narrower central region. Even then, computation of stress and strain is not
trivial. Some success has been achieved by casting ring samples and
draw
'
these over
rods mounted top and bottom (McEvoy et al.,
1985
). However, for very friable or
'
pulling
'
'
'
weak
samples, failure may occur simply in removing the sample from the mould or in
fixing it
to the instrument. An alternative approach uses a
geometry.
Experiments can also be performed using the same apparatus as for small-deformation
oscillatory or steady shear measurements, provided the sample can be made to adhere so
'
shear sandwich
'
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